Method and device for producing a tear line in a planar workpiece along a predetermined contour by material removal by means of a laser

Abstract

A method and device for producing a tear line on a planar workpiece by material removal by means of a pulsed laser, wherein machining cycles are preceded by a method step for generating and storing a reference signal curve which is formed by reference signals, causing the pulse amplitude of the laser pulses, via removal locations along the contour of the tear line. The achievement of a respective predetermined percentage of the reference signal or of an absolute distance from the reference signal by a measurement signal is used as the space-resolved deactivation criterion for each removal location, which measurement signal is obtained from a transmitted part of the pulse amplitude at the respective removal location.

Claims

1. A method for producing a tear line on a planar workpiece by material removal, said planar workpiece having a visible side and a reverse side opposite said visible side, comprising: generating a pulsed laser beam from a laser generator, said pulsed laser beam having pulses of an energy determined by a pulse amplitude and a pulse length; introducing said pulses sequentially, in machining cycles repeated more than once, along a predetermined contour for the tear line at a respective removal location from the reverse side of said workpiece, wherein material removal is performed for each removal location to a respective predetermined residual wall thickness, wherein said wall thickness may be zero; terminating, upon reaching a deactivation criterion correlating with the respective predetermined residual wall thickness, the introduction of the laser pulses in a space-resolved manner with respect to the removal locations, generating a measurement signal curve for each machining cycle, which is formed by measurement signals via the removal locations, which are each caused by detection of a transmitted part of the pulse amplitude of one of the laser pulses by means of at least one sensor of an array of sensors for each removal location; generating and storing a reference signal curve as a step temporally preceding said machine cycles, said reference signal curve being formed by reference signals causing the pulse amplitude via the removal locations with no planar workpiece being arranged between the laser generator and the array of sensors, the pulse amplitude of the laser pulses being kept constant in the steps of generating and storing said reference signal curve temporally preceding said machining cycles as well as in the machining cycles, and the achievement of a respective predetermined percentage of the reference signal or of a predetermined distance from the reference signal by the measurement signal being used as the space-resolved deactivation criterion for each removal location.

2. The method according to claim 1, wherein said laser pulses have a shorter pulse length during a first one of the machining cycles than during subsequent ones of the machining cycles, so that the energy of the laser pulses is so low that no material removal results.

3. The method according to claim 2, wherein a measurement signal equal to the reference signal is already generated for some of the removal locations in the first machining cycle, and no further laser pulses are introduced at these removal locations in subsequent machining cycles.

4. The method according to claim 2, wherein a measurement signal which is greater than the respective predetermined percentage of the respective reference signal or whose distance from the reference signal is smaller than the predetermined distance from the reference signal is already generated for some of the removal locations in the first machining cycle, and no further laser pulses are introduced at these ones of the removal locations in subsequent machining cycles.

5. The method according to claim 2, wherein a measurement signal which is smaller than the respective predetermined percentage of the respective reference signal or whose distance from the reference signal is smaller than the predetermined distance from the reference signal is generated for some of the removal locations in one of the machining cycles, and in subsequent machining cycles laser pulses are introduced at these ones of the removal locations with a shorter pulse length than those introduced at the removal locations for which no measurement signal has been formed yet in this one machining cycle.

6. The method according to claim 1, wherein the step temporally preceding said machining cycles is carried out only once for machining workpieces of the same type with the same contour for the tear line and is then used for producing the tear line in further workpieces of the same type.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the Figures:

(2) FIG. 1 shows a device for carrying out the method of the invention;

(3) FIG. 2 shows a reference signal curve, formed by reference signals via the removal locations, during the absence of a workpiece above an array of sensors;

(4) FIG. 3 shows a first measurement signal curve, formed by measurement signals via the removal locations, while an unprocessed workpiece is present above the array of sensors; and

(5) FIG. 4 shows an n-th measurement signal curve, formed by measurement signals via the removal locations, while the workpiece is being machined in the n-th machining cycle.

DESCRIPTION OF THE EMBODIMENTS

(6) FIG. 1 shows a schematic diagram of a device according to the invention.

(7) The device includes a laser generator 1 which emits laser pulses 1.1 with a pulse amplitude (pulsed laser beam), a laser scanner 2 which guides the laser pulses 1.1 within a work field A to removal locations a.sub.1, . . . , a.sub.n, and an array of sensors 3 in which sensors 3.sub.1, . . . , 3.sub.m are arranged in the form of a matrix below the work field A and each have an aperture angle sufficiently large for at least one of the sensors 3.sub.1, . . . , 3.sub.m to detect the pulse amplitude or part of the pulse amplitude of a laser pulse 1.1 impinging on any of the removal locations a.sub.1, . . . , a.sub.n within the work field A. The specific design and arrangement of the sensors 3 allows a transmitted part of the pulse amplitude to be detected from any point within the work field A and, thus, at any removal location a.sub.1, . . . , a.sub.n so that this device can be used without having to be configured for a specific tear line contour. In order to record a reference signal curve f(R.sub.a) according to the invention, the device is operated without insertion of a workpiece 4, with the laser scanner 2 being controlled such that the laser pulses 1.1 impinge on removal locations a.sub.1, . . . , a.sub.n along a predetermined contour of a tear line.

(8) For machining, a planar workpiece 4, which has a reverse side 4.1 and a visible side 4.2, is arranged in the device such that its reverse side 4.1 coincides with the work field A and its visible side 4.2 faces the array of sensors 3.

(9) As in the prior art, the material removal for producing a tear line in a planar workpiece 4 is generally effected by a laser beam directing laser pulses 1.1 onto the reverse side 4.1 of the planar workpiece 4 and being guided several times, i.e. in several machining cycles, along a predetermined contour for the tear line over the planar workpiece 4, advantageously by means of a laser scanner 2. In this case, the laser pulse 1.1 impinging on each respective removal location a.sub.1, . . . , a.sub.n causes an energy input which leads to ablation of the material of the planar workpiece 4 at the respective removal locations a.sub.1, . . . , a.sub.n.

(10) The multiple repetition of the scanning movement occurs until a respectively desired residual wall thickness, which may even be zero, has been achieved for all removal locations a.sub.1, . . . , a.sub.n. The laser generator 1 generating the laser pulses 1.1 is controlled such that laser pulses 1.1 impinge on the removal locations a.sub.1, . . . , a.sub.n only until the desired residual wall thickness has been achieved, i.e. space-resolved deactivation of the laser beam occurs during the scanning movement. The deactivation criterion is generated as a function of a part of the pulse amplitude of the laser pulse 1.1 transmitting at the respective removal location a.sub.1, . . . , a.sub.n. For this purpose, the array of sensors 3 is arranged on the visible side 4.2 of the planar workpiece 4, opposite the action of the laser beam. This is as far as the method is known from the prior art.

(11) The method according to the invention is novel insofar as it obtains a deactivation criterion which correlates with the respectively desired residual wall thickness. Obtaining the deactivation criterion according to the invention results in a narrower tolerance of the residual wall thickness, spatially resolved via the removal locations a.sub.1, . . . , a.sub.n along the contour, and thus in a reproducible tearing behaviour of the tear line.

(12) Due to the deactivation criterion being obtained by a comparison between a space-resolved reference signal curve f(R.sub.a) and measurement signal curves f.sub.1(M.sub.a), . . . , f.sub.n(M.sub.a), there is no need to adapt the arrangement of the sensors 3 to the contour of the tear line. This makes it possible, using a device which comprises a fixed array of sensors 3, to produce workpieces 4 with different contours or workpieces 4 with identical contours located at different relative positions within the work field A.

(13) For this purpose, the method is changed as follows:

(14) The above-described machining cycles, wherein for each laser pulse 1.1 a material removal generally occurs at one of the removal locations a.sub.1, . . . , a.sub.n, are temporally preceded by a method step wherein a reference signal curve f(R.sub.a) is formed from reference signals R.sub.a, e. g. in the form of a voltage, resulting from the pulse amplitudes, via the removal locations a.sub.1, . . . , a.sub.n and is stored. For this purpose, the laser beam is guided once along the contour of the tear line, emitting one laser pulse 1.1 per removal location a.sub.1, . . . , a.sub.n. However, there is no workpiece 4 in the work field A above the array of sensors 3. The sensitivity of the sensors 3.sub.1, . . . , 3.sub.m is adjusted, e. g. by means of attenuators, such that the pulse amplitude of the laser pulses 1.1 is within the dynamic range of the sensors 3.sub.1, . . . , 3.sub.m. Depending on the position of any one of the removal locations a.sub.n with respect to the sensors 3.sub.1, . . . , 3.sub.m, either one of the sensors 3.sub.1, . . . , 3.sub.m or several of the sensors 3.sub.1, . . . , 3.sub.m will detect the pulse amplitude or parts of the pulse amplitude of the laser pulse 1.1, from which a reference signal R.sub.a is formed.

(15) For the sake of simplicity, reference shall be made hereinafter to a reference signal or a measurement signal R.sub.a or M.sub.a, respectively, regardless of whether only a single one of the sensors 3.sub.1, . . . , 3.sub.m detects the pulse amplitude or a part of the pulse amplitude and a single reference signal or measurement signal R.sub.a or M.sub.a, respectively, is derived therefrom, or whether several of the sensors 3.sub.1, . . . , 3.sub.m respectively detect a part of the pulse amplitude, from which several individual reference signals or measurement signals R.sub.a or M.sub.a, respectively, are derived, which are used to form a total reference signal or a total measurement signal, e.g. by addition, an OR link or averaging.

(16) The sensors 3.sub.1, . . . , 3.sub.m are usually arranged in correlation with the contour of the tear line. In connection with carrying out the method according to the invention, however, they are advantageously arranged in the form of a matrix, covering a work field A over which the laser beam can be generally guided, so that different contours of tear lines located within the work field A can be produced by the same configuration of the array of sensors 3.

(17) If a laser scanner 2 is used to guide the laser beam, the impingement angle of the laser beam changes according to the position of the current removal location a.sub.1, . . . , a.sub.n in the work field A from 0 at the centre to approx. 20 at the edges. This results, for each contour of a tear line in the work field A, in an individual irradiation direction for the removal locations and, thus, for the sensors 3.sub.1, . . . , 3.sub.m and in individual distances of the removal locations a.sub.1, . . . , a.sub.m, from the nearest sensor centres. Depending on the respective aperture angle of the sensors 3.sub.1, . . . , 3.sub.m, for signal detection, and on the distance of the sensors 3.sub.1, . . . , 3.sub.m from each other, a reference signal R.sub.a results for each removal location a.sub.1, . . . , a.sub.n that may differ according to the respective distance of the removal locations a.sub.1, . . . , a.sub.n from the sensors 3.sub.1, . . . , 3.sub.m and the impingement angle of the laser beam, because the sensors' sensitivity decreases as the distance from the sensor centre increases.

(18) The reference signals R.sub.a are stored as signals assigned to the individual removal locations a.sub.1, . . . , a.sub.n.

(19) This method step, which precedes the machining cycles, has to be repeated in order to machine workpieces 4 with different contours for the tear line. When machining workpieces 4 with the same contour of the tear line, which also have the same relative position with respect to the sensors 3.sub.1, . . . , 3.sub.m of the array of sensors 3, the reference signal curve f(R.sub.a) stored once can be used.

(20) In each of the subsequent machining cycles, a respective measurement signal curve f.sub.1(M.sub.a), . . . , f.sub.n(M.sub.a) is formed and compared to the reference signal curve f(R.sub.a). The measurement signals M.sub.a are each caused by a transmitted part of the pulse amplitude of a laser pulse 1.1 and are detected and stored e.g. in the form of a voltage assigned to the removal locations a.sub.1, . . . , a.sub.n.

(21) The laser pulses 1.1 have the same unchanged pulse amplitude both in the preceding method step and in the machining cycles. Whether material removal takes place and how much material, if any, is removed per laser pulse 1.1 at any of the removal locations a.sub.1, . . . , a.sub.n is controlled via the pulse length. If it is to be expected, due to the properties of the planar workpiece 4, that measurement signals M.sub.a are formed already in the first machining cycle, and their level corresponds exactly or approximately to the reference signal R.sub.a, the pulse length of the laser pulses 1.1 is advantageously selected to be shorter in this first machining cycle than in the subsequent machining cycles so as to reduce the energy input to the extent that no material removal occurs yet.

(22) Such workpieces 4 may be tissues or fabrics with a perforated structure, so that the laser pulses 1.1 can already pass through the workpiece 4 unhindered at some of the removal locations a.sub.1, . . . , a.sub.n without prior material removal. The method is generally applicable to any workpiece 4, regardless of whether measurement signals M.sub.a are detected in a first machining cycle for all removal locations a.sub.1, . . . , a.sub.n for some of the removal locations a.sub.1, . . . , a.sub.n or for none of the removal locations a.sub.1, . . . , a.sub.n. What is decisive is that the deactivation criterion used for the individual removal locations a.sub.1, . . . , a.sub.n is in each case a predetermined percentage of the reference signal R.sub.a or an absolute distance from the reference signal R.sub.a and machining is terminated at the respective removal location a.sub.1, . . . , a.sub.n if a measurement signal M.sub.a is formed for the first time for said removal location a.sub.1, . . . , a.sub.n that is greater than or equal to the predetermined deactivation criterion. How large the percentage of the reference signal R.sub.a or the distance from the reference signal R.sub.a selected as the deactivation criterion depends on the material properties of the workpiece 4.

(23) FIGS. 2 to 4 show a reference signal curve f(R.sub.a) and first and n-th measurement signal curves f.sub.1(M.sub.a), f.sub.n(M.sub.a). In this case, a transparent workpiece 4 was machined, so that even without any material removal at all of the removal locations a.sub.1, . . . , a.sub.n via the contour of the tear line to be introduced, a part of the pulse amplitude of the respectively impinging laser pulse 1.1 is transmitted.

(24) FIG. 2 shows a reference signal curve f(R.sub.a) resulting from the individual reference signals R.sub.a that are each assigned to one of the removal locations a.sub.1, . . . , a.sub.n. The reference signals R.sub.a deviate more or less from a theoretical target value R.sub.soll.

(25) FIG. 3 shows a first measurement signal curve f.sub.1(M.sub.a) resulting from the individual measurement signals M.sub.a that are each assigned to one of the removal locations a.sub.1, . . . , a.sub.n during the first machining cycle. The measurement signals M.sub.a deviate more or less from the respective reference signals R.sub.a or correspond to the reference signal R.sub.a at some of the removal locations a.sub.1, . . . , a.sub.n. In the latter case, no further laser pulse 1.1 is introduced at any of these removal locations a.sub.1, . . . , a.sub.n in the further machining cycles.

(26) FIG. 4 shows a further, n-th measurement signal curve f.sub.n(M.sub.a). The n-th measurement signal curve f.sub.n(M.sub.a) has visibly approached the reference signal curve f(R.sub.a).

(27) As an example, the machining will be explained at three different ones of the removal locations a.sub.1, . . . , a.sub.n, for example three out of a total of e.g. 270 removal locations a.sub.1, . . . , a.sub.n (n=270) describing the contour of the tear line, namely a.sub.17, a.sub.113 and a.sub.241.

(28) At the removal location a.sub.17, the measurement signal M.sub.17 is still a great distance away from the reference signal R.sub.17 during the first machining cycle, while its distance in the n-th machining cycle is only marginal and machining has been terminated for this removal location a.sub.17. The situation is similar at removal location a.sub.113, where machining was terminated earlier. At removal location a.sub.241, a measurement signal M.sub.241 which corresponds to the reference signal R.sub.241 or which meets the deactivation criterion was generated already in the first machining cycle, so that advantageously no removal takes place, which is feasible by working with such a short pulse length in the first machining cycle that the energy input by a laser pulse 1.1 is below a threshold for material removal.

(29) A transparent planar workpiece 4 may be, for example, a tissue, wherein the removal locations a.sub.1, . . . , a.sub.n, simply spoken, are located either on a woven thread, a junction of woven threads or a hole bordered by woven threads, which means there are three groups of removal locations a.sub.1, . . . , a.sub.n through which different-sized parts of the pulse amplitude are transmitted. Such workpieces 4 may also be fabrics, wherein removal locations a.sub.1, . . . , a.sub.n differing in transparency result in the same manner as in a tissue.

(30) While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

LIST OF REFERENCE NUMERALS

(31) 1 laser generator 1.1 laser pulse 2 laser scanner 3 Array of sensors 3.sub.1, . . . , 3.sub.m 3.sub.1, . . . , 3.sub.m sensor (of the array of sensors 3.sub.1, . . . , 3.sub.m) 4 workpiece 4.1 reverse side 4.2 visible side A work field a.sub.1, . . . , a.sub.n removal location f(R.sub.a) reference signal curve f.sub.1(M.sub.a) first measurement signal curve f.sub.n(M.sub.a)n-th measurement signal curve M.sub.a measurement signal R.sub.a reference signal R.sub.soll target value